1. Field of Use
This invention relates generally to a refrigeration system comprising an adjustably controllable expansion valve connected in a refrigerant supply line from a receiver to an evaporator. In particular, it relates to an improved electronic control system for adjusting said expansion valve in accordance with refrigeration system conditions.
2. Description of the Prior Art
A typical prior art refrigeration system generally comprises a motor-driven compressor to compress gaseous refrigerant, a condenser in which compressed gas from the compressor liquifies, a receiver for the liquid refrigerant, an evaporator in which the liquid evaporates prior to returning to the compressor, and an adjustably controllable expansion valve connected in the liquid refrigerant supply line from the receiver to the evaporator. Such a refrigeration system also usually comprises control means or system to adjust the expansion valve so as to control superheat and thereby maintain the maximum amount of liquid refrigerant in the evaporator for cooling efficiency while preventing possibly damaging flow of liquid refrigerant to the compressor. U.S. Pat. No. 3,577,743 issued May 4, 1971 to Long and assigned to the same assignee as the present patent application discloses one type of such a prior art rtefrigeration system and expansion valve control means. That prior art control means employs first and second temperature sensors or probes located at two widely separated points in the system i.e., in the refrigerant inlet port and in the refrigerant outlet port of the system evaporator to control the superheat condition of the system refrigerant. One temperature sensor senses the temperature of mixed liquid and gaseous phase refrigerant near the evaporator inlet while the other temperature sensor senses the temperature of gaseous phase refrigerant near the evaporator outlet. The two temperature sensors are connected to a temperature differential determining means which regulates the system expansion valve in accordance with a desired differential temperature or superheat condition between the liquid and gaseous phase refrigerant. The sensor at the inlet senses the saturation temperature of the refrigerant at the pressure existing at the inlet. The other sensor measures the temperature of the refrigerant vapor at the evaporator outlet. The difference of these temperatures is the superheat. Generally, there is a pressure drop associated with an evaporator, particularly a direct expansion type evaporator. However, the pressure drop through such an evaporator results in an incorrect superheat reading if done in the above-described manner unless other measures are taken.
To illustrate this problem, if, for example, the pressure at the evaporator inlet were 3.6 psig, this would correspond to a -20.degree. F. saturation temperature if ammonia were the refrigerant. If the above-described expansion valve superheat control were set to maintain a 10.degree. F. superheat, the temperature at the outlet would be -10.degree. F. This is a 10.degree. superheat when compared to the -20.degree. F. saturation temperature at evaporator inlet. If, however, there were a 2.8 psi pressure drop through the evaporator, a pressure of 0.8 psig (3.6-2.8) would be present at the outlet. A pressure of 0.8 psig corresponds to a saturation temperature of -26.degree. F. Therefore, at the outlet the superheat is [-10.degree. F.-(-26.degree. F.)]=16.degree. F. and not the 10.degree. F. that was set. With the greater amount of superheat, the system is not making full use of available evaporator capacity. This is one disadvantage of the afore-described prior art system. There are various solutions available to circumvent this shortcoming, however, all are costly. However, just setting the expansion valve control lower to compensate for pressure drop can also create problems. If, for instance, in the foregoing example the controller was set for 4.degree. F. superheat to compensate for the 6.degree. F. lost due to pressure drop, then a problem arises due to an evaporator load reduction when the refrigerant flow is reduced. Of course, with reduced refrigerant flow, there is reduced pressure drop and less need for 6.degree. F. compensation. Trying to compensate for pressure drop in this manner is not an ideal solution.
Another problem with the above-described prior art control system centers around start-up. The control system is designed such that when a superheat that is higher than set point is reached, a signal is sent to the adjustable expansion valve which begins to open and feeds more liquid refrigerant to the evaporator. The expansion valve continues to modulate until it is positioned to maintain the set point. At start-up after a prolonged shutdown (one day, for example), the two temperature sensors are at the same temperature thereby indicating 0.degree. superheat. When the refrigeration system is started, the expansion valve remains closed, not allowing liquid refrigerant into the evaporator, and without allowing liquid into the evaporator superheat will not be established, nor will any cooling occur. To correct this problem, a timer is used to electrically force the expansion valve to open for a period of time, ideally long enough to establish liquid flow and a superheat condition, but not so long as to overfeed the evaporator and allow liquid refrigerant to enter the compressor. With any number of variables affecting the time period needed to establish liquid flow, use of a timer also is not an ideal solution.
The prior art also contains U.S. Pat. No. 4,375,753 issued Mar. 8, 1983 to Imasu et al and showing the use of temperature sensing thermistors at various points in refrigeration systems to effect expansion valve control.
It is desirable, therefore, to provide improved control systems for expansion valves in refrigeration systems to avoid or overcome the aforesaid problems.